File Formats
There are several things you must take into consideration when trying to play music on your Nintendo DSi. First, the files cannot be copy protected or else they will not play. Some songs from services such as iTunes are copy protected, so keep that in mind. Also, the songs must be in the AAC format, with the .m4a, .mp4 or .3gp extension, for the DSi to play them. The popular MP3 format will not work.
Converting Files to AAC Format
Many of your songs may not be in the supported AAC format and Nintendo was aware of this. This is why Nintendo officially teamed up with Nero to provide a simple way to convert your music to the supported AAC format. If you already have this or another converter, you can simply use it to convert your songs to an AAC format.
Transferring Files
Transferring music to your Nintendo DSi is as simple as dragging and dropping. You will need an SD or SDHC card and a computer that has an SD card slot or a card reader. Once the card is inserted into your computer, you simply drag the compatible files to the card. Once the songs are on your card, insert it back into the DSi and open Nintendo Sound. The songs should be there and allow play.
Troubleshooting
If the songs you placed in your DSi are in the correct format but still will not play, Nintendo asks that you test your device first. Download the supplied "test.mp4" file from the support site and see if that will play on your device. If it does not play, you can contact Nintendo directly regarding the issue at 1-800-255-3700.